Conventionally, there are many types of methods of coding image signals including DPCM (Differential Pulse Code Modulation) or DCT (Discrete Cosine Transform) and so forth. In particular, the latter is well known for high efficiency in coding of natural images such as a scenery and adopted in an international standard on multimedia coding such as JPEG (Joint Photographic Coding Experts Group) or MPEG (Moving Picture Experts Group).
Meanwhile, with recent improvement of computer technologies and image processing techniques, composite images resulting from compositing plural images using chroma key processing or computer graphics have been widely used. The composite image includes an edge whose pixel value changes sharply and has characteristics which significantly differ from those of conventional natural image, e.g., high spatial resolution.
In coding of the above-described composite image, if image signals corresponding to images to be composited are coded together with signals indicating a composite ratio for each image, decoding side can perform decoding process for a desired image among plural images constituting a composite image as well as decoding process for the composite image itself. In this case, the resultant composite image is effective in interactive multimedia (interactive processing to plural images related to each other and provided simultaneously), but a special coding process for coding the signals indicating a composite ratio is required.
Signals indicating this ratio of composition represent a degree of transparency of a background with respect to a foreground when a prescribed image and the other image are assumed to be a foreground and a background, respectively, for formation of a composite image. The signals indicating composite ratio are set for each image and referred to as transparency signals to distinguish them from pixel value signals used for color display of images.
More specifically, transparency signals of individual images (objects) constituting a composite image to be displayed on a display screen have a characteristic in which values thereof are almost equal inside the objects and vary greatly around the boundaries thereof, which characteristic is different from that of pixel value signals for displaying natural images; in a prior art coding method, however, coding is performed to transparency signals using the coding method for pixel value signals indicating a pixel value of an image and the same parameters for coding.
In addition, in coding of image signals, image signals of higher image quality require more amount of code to be used for the coding process. In the decoding side, since one image (object) is regenerated by decoding pixel value signals and transparency signals together, an image quality of a regenerated image is degraded if an image quality of a regenerated image formed by one type of signals is high image quality and that of the regenerated image formed by the other type of signals is low.
When performing data coding process, in general, high coding efficiency is not obtained unless a coding method corresponding to a quality of data to be coded is used. However, in the prior art coding method, the coding method and parameters used in coding process for pixel value signals are also used in coding process for transparency signals, which causes problems including an unnecessarily high image quality of transparency signals in comparison with that of pixel value signals and the coding method being not suitable for coding process for transparency signals, resulting in that an enormous amount of code is required for coding process for transparency signals.
The present invention is introduced to resolve the above-described problems, and its object is to provide an image coding apparatus and an image coding method capable of optimally controlling a coding method and a coding parameter for transparency signals according to a coding parameter for pixel value signals or independently of the coding parameter for pixel value signals.
It is another object of the present invention to provide an image decoding apparatus and an image decoding method capable of properly decoding coded transparency signals resulting from optimal coding process for transparency signals in accordance with the coding parameter for pixel value signals or optimal coding process for transparency signals independently of the coding parameter for pixel value signals.
It is still another aspect of the present invention to provide a quantization control method which is suitable for an optimal coding process for transparency signals according to the coding parameter of pixel value signals and an inverse quantization control method which is suitable for a decoding process for the coded transparency signals acquired from the coding process.
It is further object of the present invention to provide a data recording medium which stores a program for implementing in a computer an optimal coding process for transparency signals on the basis of the coding parameter of pixel value signals or independently of the coding parameter of pixel value signals, a decoding process for the coded transparency signals resulting from the coding process, and a quantization process which is suitable for the coding process and an inverse quantization process which is suitable for the decoding process.